This invention relates to a process for producing a high tensile strength, light gage sheet from an aluminum-magnesium alloy containing a relatively low percentage of magnesium, i.e., between about 1.5 to about 2.3% by weight; the light gage sheet having a strength in excess of 50,000 psi when in a fully-hardened condition.
In general, aluminum-based alloys have found acceptance in the marketplace because of their light weight and relatively low cost. However, at least the latter attribute has been difficult, if not impossible, to achieve in a high-strength aluminum-based material. Aluminum-magnesium alloys have been used to satisfy strength requirements; however, the strength requirement of 50,000 to 60,000 psi for certain products, such as rigid containers and end covers for rigid containers made of aluminum, aluminum alloy, steel or other material, as well as products having similar strength requirements, are obtainable with an aluminum-magnesium alloy containing 5% by weight of magnesium which not only raised the cost of the product but presented more difficulties in casting and producing light gage material.
In an attempt to more efficiently make such high strength aluminum-magnesium alloy material, continuous strip casting was tried but it became apparent that aluminum alloy containing a high percentage, i.e., about 5% by weight, of magnesium could not be continuously cast at commercially acceptable speeds. Furthermore, prior processes involving continuous casting of an alloy having a magnesium content of about 2% require a full or partial anneal before cold rolling and/or one or more intermediate annealing steps during cold rolling that precipitate some of the dissolved constituents and soften the metal for ease in subsequent cold working. Consequently, the highest possible strength of the resulting product, consistent with desired quality, has been between about 45,000 to 46,000 psi in the full hard condition. Furthermore, conventional practice for producing a sheet of 5051 alloy, containing about 1.9 magnesium, with a temper of H19 involves a heat treatment at about 675.degree. F. for a period of two hours at 0.07 gage in order to obtain a strength of about 41,000 psi, maximum.
Pursuant to the present invention, a process is provided for producing a cold reduced, high tensile strength, aluminum-magnesium alloy sheet from a low magnesium content starting composition having a yield and full-hardened tensile strength as high as 50,000 psi to about 60,000 psi which is the strength of products produced from high magnesium content aluminum-magnesium alloys in accordance with conventional practices.